Process for the concentration of isotopes



3 w. A. WEBB 2, 37,430

PROCESS FOR THE CONCENTRATION OF ISOTOPES Filed May 15, 1934 FIE- . LUta H INVENTOR. I l 1 We//5 A. M4265 ATTORNEY Patented Nov. 2 2, 1938UNITED STATES PATENT OFFICE raocsss on THE comes-marrow or The presentinvention relates to the recovery of isotopes,-and has for its principalobject the provision of an improved apparatus and method for eiiectingtheir separation.

It is now known that certain elements exist in isotopic forms ofdiflerent atomic weight, and extensive studies are being carried on todetermine the chemical and physical properties of the heavier isotopes,the rarity of which, heretofore, has permitted them to escape detection.Examples of these heavy isotopes areiound in hydrogen, which is nowknown to have atleast one isotopic form having an atomic weight of two,and in oxygen. which is now known to have at least one isotopic formhaving an atomic weight of eighteen.

In order to eflect separation of the various isotopic forms of what hasheretofore been assumed to'be a single element, it is, of course,necessary to rely on some diflerence in the physical or chemicalbehavior of the several iorms either in elemental or compound form.Several techniques for such separation have been proposed which includediilerential diffusion of gaseous forms through metallic or othermembranes, fractional distillation of liquefied gases and liquidcompounds, and fractional electrolysis of various compounds. The firsttechnique depends upon the fact that the lighter isotopes diffusethrough membranes more readily than the heavier, the second, upon slightdiflerences in boiling points, and the third, upon the fact thatcompounds of the lighter isotopes are more readily decomposed byelectrolysis than the' heavier.

The improvements of the present invention are principally directed tothe method of separation by fractional electrolysis although certain oftheir principles are equally applicable to other methods. Brieflysummarized, the objects of the invention are as follows:

The provision of a method and apparatus for obtaining graduallyincreasing concentrations of the isotopes in a series of concurrentlyoperating stages of a concentrator;

The provision of a method and apparatus for concurrently concentratingisotopes of a plurality of elements and separating the maximumconcentrations of each;

The provision of a method and apparatus in which, during a process ofisotope concentration, additional material of equal isotopicconcentration may be introduced into the material undergoingconcentration;

The provision of controlling means whereby the concentration of materialintroduced, as. above, is maintained equal to that of the materialundergoing concentration, thus avoiding loss of efficiency by dilution;

The provision of a method and apparatus 10 whereby isotope concentrationmay be effected in a batch oi material without change in its volume; Theprovision of an electrolyte particularly -adapted for the electrolyticconcentration of the electrolytic separation process;

Figure 2 is a diagrammatic section of the catalyzer; and

Figure 3 is a diagram of ancillary apparatus for the separation ofconcentrated mixed isotopes.

In the following description, the recovery by electrolysis of theheavier isotopes of hydrogen and oxygen from wate w'ill'be described, asa convenient example of he application oi! the invention, it beingunderstood that it is equally applicable to other substances containingone or both of the heavy isotopes. The hydrogen isotope having an atomicweight of two, will be referred to as deuterium, and under the chemicalsymterium, and a concentration formula may be r calculated for givenconstant temperatures, current densities, and other factors hereinafternoted, as follows:

Where .Ho is the amount of water containing ordinary hydrogen, beforeelectrolysis;

Do is the amount of water containing deuterium, before electrolysis;

H is the amount of water containing ordinary hydrogen, afterelectrolysis;

D is the amount of water containing deuterium,

after electrolysis; and

is a factor representing the difference between the rate of electrolyticdecomposition of the hydrogen oxide and the deuterium oxide,- the factorbeing less than unity in the above formula, since the deuterium oxidedecomposes less readily.

Under the conditions set forth in the following description, a value forof about .143 has been obtained, from which it follows that if 1000grams of water containing 25% D20 is reduced to 100 grams of H20 plus anunknown quantity of D20, the concentration of deuterium thus effectedmay be calculated as follows:

This apparatus comprises a circulatory system containing a quantity ofwater having an electrolyte in solution and consisting of a cell I, thewall of which serves as an anode, and which has a cathode '2 locatedtherein.

Decomposition of the electrolyte solution produces a mixture of thegaseous products of the decomposition with a portion of the solution,which mixture passes upwardly through a tube 3 through cooling coils 4,surrounded by a. jacket 5 through which acooling medium'is passed, andinto a separator 6 having means such as a baflie plate 1 therein toinsure separation of the mixed gases from the liquid.

From the separator i, the liquid returns to the cell i through tube 8connected to tube 9 which passes through cooling coils I 0 similar tocoils l and correspondingly jacketed and cooled. From these coils, theliquid passes into a reservoir ll having a cock I2 by means of which theair pressure in the reservoir maybe regulated to maintain proper liquidlevels in the system. Also connected to this reservoir II is a tube l3,adjustable so that its end may be maintained below the level of theliquid in the reservoir, and connected with the cell I by means of tubel4 havinga cock II, by means of which the cell may-be drained. Theevolved gases, acting to raise the liquid in tube 3, maintain the liquidin this system in constant circulation, cooling and mixing itadequately.

From the separator E3, the mixed gases pass into tube it and through atrap ll provided with a heme plate iii and a drain cook it, which trapis preferably provided for the purpose of separating any foam escapingfrom the separator. From this trap, the mixed gases are conveyed, bytube' 2%, to a recombining means 26, hereinafter described in detail,which effects a chemical recombination of the gases and feeds theresulting liquid and/or vapor into a condenser 22, of conventional form,whence the liquid is conveyed by a tube 9' to the reservoir H of anadjacent circulatory system or stage" corresponding in construction andoperation to the system just described. It will be evident that as manyof these systems may be thus interconnected as are necessary to effectthe degree of concentration desired.

The separation of the isotopes may be considerably expedited by the useof an electrolyte which does not contain the element sought to beseparated in its composition. If, for instance, potassium hydroxide, isused as an electrolyte in the concentration of deuterium, the K01)concentration of the KOH will increase as rapidly as the D20concentration of the H20 and, if a full yield is to be obtained, it willbe necessary to free the deuterium from the potassium salt by bubblingcarbon dioxide through the electrolyte or by equivalent means. Thisextra operation may be avoided by using a hydrogen free electrolyte suchas sodium or potassium carbonate, in a concentration giving the minimumelectrical resistance.

It has been found that the separation factor a may be favorably affectedby the use of a cathode material requiring a relatively high overvoltagein operation, and since the objective is not a maximum evolution of gasfor a, given amount of current, but rather a maximum separation of theisotopes, it is desirable to use a material such as lead, aluminum orother material requiring a comparatively high overvoltage for thecathode.

Certain of these metals, aluminum for instance, are attacked by alkalineelectrolytes, but since the reaction is checked by cooling theelectrolyte solution, and since such cooling also affects the separationfactor favoraby, it is evident that the cooling of the solution isdesirable for the double purpose of inhibiting electrode decompositionand favorably modifying the separation factor.

Where it is desired to concentrate only one of a plurality of isotopespresent, the factor or as to this isotope may be favorably affected byincreasing the current density at the electrode evolving it, as by usinga rod of comparatively small area for such an electrode, as comparedwith a cylindrical chamber, forming the cell wall, for the otherelectrode.

The incidental concentration of heavy oxygen which takes place duringdeuterium concentration is undesirable unless the heavy oxygen beafterward separated onto! the yield, as hereinafter described, becauseit eflects an increase in the density of the yield which may beerroneously attributed to the presence of a larger concentration ofdeuterium. By controlling the relation of the current density on the twoelectrodes as above described such incidental concentration of heavyoxygen may be minimized- The catalyzer In effecting the recombination ofevolved hydrogen and oxygen, heretofore referred to, it is desirable tolessen the force of the reaction, which under ordinary circumstanceswould be explosive. The present invention contemplates the provision ofa novel form of catalyzer for this purpose, such as that shown in detailin Figure 3.

The catalyzer comprises a tube 25, preferably of metal, such as brass,and sealed at both ends except for the access provided by inlet tube 26and outlet tube 21 Adjacent these tubes stuflings 28 of fibrous or othermaterial through which the gases and vapor may pass freely arepreferably provided, and theremainder of the tube is filled with inertmaterial such as sand, of an 8 to 10 mesh size through the intersticesof which the gases and vapors may pass in intimate contact with thesurfaces of the particles. These particles should be free from sharppoints and protuberances which might be heated disproportionately duringthe reaction and thus cause the reaction to be accelerated to anexplosive degree. Upon the surfaces of these particles, platinum or anequivalent catalytic agent is deposited in concentrations varying from aminimum at the inlet end of the tube to a maximum at'the outlet.

Such deposition of platinum upon sand may be efiected by dissolvingplatinum in aqua regia, soaking clean sand in the solution and heatingthe wet sand to effect decomposition of the'chlorplatinic acid leavingmetallic platinum uniformly deposited upon the sand particles. Theremaining solution then may be diluted and additional sand soakedtherein and heated as before, in order to secure successive deposits ofsmaller platinum concentration.

' For a catalyzer tube of three quarter inch inside diameter,satisfactory concentrations of platinum on sand have been secured as.follows. First, enough clean sand to fill one inch of the tube wassoaked in a solution of .0371gram' of platinum in 1 c. c. of aqua regia,and, after heating, this sand was placed in the portion 29 of the tubeadjacent the outlet tube 21. Next, an equal amount of platinum wasdissolved. in 2 c. c. of aqua regia, and enough sand to fill anotherinch of the tube was similarly treated with this solution and placed inthe portion 30 of the tube 25. Next an equal amount of platinum wasdissolved in 10 c. c. of aqua regia and sand similarly treated thereinwas usedto fill the next inch 3| of the tube 25. Next an equal amount ofplatinum was dissolved .in 25 c. c. of aqua regia and sand similarlytreated therein was used to fill the next inch 32 of the tube 25. Nextan equal amount of platinum was dissolved in 3'7 c. c. of aqua regia andsand similarly treated therein was used to flll the next inch 33 ofthe'tube 25. Next, an equal amount of platinum was dissolved in 50 c. c.of aqua regia and sand similarly treated, therein was used to fill thenext three inches 343 of the tube 25. Next an equal amount of platinumwas dissolved in 100 c. c. of aqua regia and sand similarly treatedtherein was used to fill the next two inches 35 of thetube 25. Next, anequal amount of platinum was dissolved in 200 c. c. of aqua regia andsand similarly treated therein was used to fill the next inch 36 of thetube 25. Finally, an equal amount ,platin'um was dissolved in 300 c. c.of aqua regia 1 and sand similarly treated therein was used to an,thenext inch 31 ofthe tube 25. The foregoing is it accelerated to anexplosive rate.

'tion at the optimum value originally set.

centrations which will produce an operative device, and it is evidentthat departures from the r exact concentrations set forth may still bewell within the scope of the invention which contemplates a catalyzer ofthe characteristics hereinafter set forth.

In operation, it is desirable to mount the tube 25, as shown in Figure1, in an insulated housing 40 which may contain a plurality of suchcatalyzers, and to preheat it to an efllcient operating temperature bymeans of electrical resistance coils 4| or equivalent means. Some heatis generated by the reaction, and hence the insulation should bedesigned to maintain the tubes at an operating temperature of about 200C. either with or without external heating from the means 4i.

Mixed gases entering through the inlet tube 26 first impinge upon thesurfaces of the sand particles in the portion 31 of the tube 25, but theplatinum concentration there is so low that the temperature riseengendered by the reaction is insufliclent to accelerate the reaction toan explosive rate. Any concentration below this limit will besatisfactory, the particular concentration hereinbefore set forth beinggiven merely as an operative example. As the mixture of the gases andwater vapor passes along the tube 25 toward theoutlet tube 21,successively increasing concentrations of platinum are encountered, butthe gases are also increasingly diluted by water vapor which tends toslow the reaction so that in no case The concentration of platinumadjacent the outlet tube '21, however, is sufllcient to insure effectivecombination of substantially all of the hydrogen and oxygen present.

A catalyzer of the proportions described above has been used torecombine the gases evolved from water by a ampere current, withoutaccelerating the reaction to an explosive rate.

The process, and operation of the apparatus In the final stages ofconcentration for the purpose of obtaining deuterium oxide in as pure astate as is possible, the apparatus shown in Figure l is preferablyoperated with an equal current in each cell so that the volume of liquidin each cell system or stage except the last, shown at the right in thisfigure, remains constant, due to the fact a that it receives as muchliquid from the catalyzer of the adjacent system as it decomposes.

This avoids the necessity for distillation of the electrolyte solution,which becomes desirable whenever major changes in its volume takesplace. In the stages or systems in which-the volume remainsconstant, nosuch distillation is necessary, since the equality between input andoutput automatically retains the electrolyte concentra- As the volume ofliquid is reduced in the last stage A.

which receives-no input, it is desirable to distill the liquid atintervals to remove enough electrolyte to prevent supersaturation of thesolution.

In this type of operation it will be evident that the isotopeconcentration of stage B will be built up during operation byelectrolytic passing off of the lighter isotope to stage C and by thereception of liquid from stage A having a substantial isotopeconcentration. At the commencement of operation the isotopeconcentration of the liquid in stage B should be equal to that receivedvia the catalyzer from stage A. As the catalyzers output increases inisotope concentration due to the volume reduction taking place in stageA, the

all

electrolytic action in stage B will concurrently build up its ownisotope concentration to finally equal the initial concentration ofstage A.

In a similar manner stage C and the remaining subordinate stages willeach increase in isotope concentration so as to equal-the initialisotope concentration of the next rightward stage by the time stage Ahas been reduced in volume to the point where such concentration attainsthe ,maximum desired.

When this point is reached, the liquid is drained from stage A by meansof the cock I, and distilled. This is the deuterium oxideyield. Stage Bnow becomes stage A for further operations, the former stage A beingfilled with new water of the concentration proper for its alphabeticalposition in the stages, and its catalyzer output shut 011 from originalstage B by means of two-way valve 50, and diverted into header II forany desired purpose. To facilitate such operation of the apparatus, theseveral stages are preferably arranged in an endless chain rather thanin line, as shown for convenience in the drawing.

The foregoing type of operation may, of course be used for allconcentration work, but is particularly desirable in the moreconcentrated stages because it avoids the losses incident to frequentdistillations of the electrolyte solution.

The apparatus may also be operated so as to obtain a reduction in thevolume of liquid in each stage, this effect being obtained by connectingthe electrodes of the several stages in parallel and controlling thecurrent input to each cell by a rheostat, by the proportionlng of theelectrodes or by both such means.

When operating in this way, the liquid in stage A will be reduced involume from, say, 100 c. c. to c. 0. while the liquid in stage B,subjected to a correspondingly heavier current will be reduced from 1000c. c. to 100 c. c. and that in stage C, subjected to a correspondinglystill heavier current, from 10,000 0. c. to 1,000 c. c. The currentincrease for each successive stage must be great enough, of course, toprovide for the redecomposition of water received from the catalyzer ofthe adjacent stage in addition to effecting the proportionately greatervolume reduction planned for the stage.- Figure 1 may be taken only asdiagrammatic for this type of operation since it will be evident thatthe circulatory systems of the several stages must be proportioned insize to the volumes of liquid to be handled, which may, of course, be inany desired ratio to each other.

A lesser volume reduction may be effected in this method of operation byfeeding recombined gases from stage A to stage C, from stage 3 to stageD, and from stage C to stage E, lettering the stages in the order inwhich they become yield cells. In this way, the liquid in stage C isreduced in volume during its operation as stage B before it functions asstage A, and hence the isotope concentration of stage C is more easily.brought into equality with the concentration of the isotope in thegases evolved from stage A.

This plan of operation possesses the advantage of effecting aproportionately greater concentration within fewer stages than thatheretofore described, and is therefore. more useful in the earlyconcentration procedure than in the final purification.

While the foregoing apparatus and process has been described withreference to the recovery of the hydrogen isotope, deuterium, the samemeans are applicable with very slight changes of proportion to therecovery of the oxygen isotope of atomic weight eighteen and toseparation of other isotopes. In the case of the oxygen isotope, it isknown that its abundance in nature is much greater than that ofdeuterium and it is therefore apparent that less volume reduction of theelectrolyte solution will be necessary in its recovery. However, as itsatomic weight difference from the commoner oxygen is proportionatelyless than the proportionate weight difference of the hydrogen isotopes,its separation factor in electrolysis is much more unfavorable, and manymore stages will be required in apparatus for its recovery.

Both of these considerations favor the selection of the system in whichthe liquid volume remains constant up to the last stage.

Concurrent: concentration of hydrogen and oxygen isotopes In the lightof the foregoing description, it will be seen that, during theconcentration of deuterium oxide, some concentration of the heavierisotope of oxygen takes place incidentally, and vice versa. It istherefore possible tozconstruct apparatus forthe concentration of thetwo isotopes separately, in which such incidental concentration of theother isotope in one section is used to advantage in the other.

' A diagrammatic illustration of such apparatus appears in Figure 3,inwhich cell systems designed to separate the evolved gases are shown abined gases is shown by arrow-headed lines, and catalyzers andelectrical connections are omitted from the diagram for simplicity.

In this figure, llll represents a stage of a system such as that shownin Figure 1; ll, 12, and I! are intermediate cells designed to separatethe evolved gasesand to receive liquid produced by the recombination ofgases evolved in other cells; 14 is the volume reducing cell from whichthe deuterium oxide yield is obtained; and I5 is the volume reducingcell from which the oxygen isotope yield is obtained. Cells 14 and 15are also of the gas separating type.

After the yield is removed from cells 14 and II, the liquid from cells12 and 13 is transferred into these cells, respectively, and the liquidfrom cell II is divided into cells 12 and 13. Cell H is then refilledwith the liquid from stage 10, and the apparatus of Figure 1 is reset sothat the recombined gases from cells 12 to 15 will feed into the nextfull stage instead of into the one just emptied, which is filled withnew water and is adjusted to dischargethe recombined gases into theheader 5|.

In operation, it will be seen that light hydrogen evolved from cell 14is recombined with oxygen .and fed into cell 12; light hydrogen fromcell 12,

similarly recombined, is fed into cell 1i; light hydrogen from cell II,similarly recombined is fed into cell 13; and light hydrogen from cells13 and 15 similarly recombined, is fed into stage III.

Likewise light oxygen from cell 15, is recombined with hydrogen and fedinto cell 13; light oxygen from cell 13,, similarly recombined, is fedinto cell ll; light oxygen from cell "H, similarly recombined is fedinto cell. 12; and light oxygen from cells 12 and 14, similarlyrecombined, is fed into stage 10. From stage III on, the operation is asdescribed in connection with either mode of operation of the apparatusshownin Figure 1.

The effect of this operation is to.concentrate both the hydrogen andoxygen isotopes .in the stages leading up to stage 10, and then tosepaaismso Since the flow of the light hydrogen isotope is to the rightin this figure, it is evident that the maximum deuterium concentrationwill he found in cell it, and since the flow oi the light oxygen isotopeis to the .left in this figure, it likewise appears that themaximumlconcentration oi. the heavy ozwgen isotope willbe found in cellit. The number of cells shown is, of course, merely illustrative and maybe varied at will.

It will also be apparent that the process described is not dependentupon any particular apparatus, but may be carried out by manual transferof the materials according to the pro-.-

: decomposing a plurality of batches oi material thedecompositionproducts of one of said batches to form material of different isotopeconcentra-' tion than the batch from which said products were derived,and introducing said recombined decomposition products into another ofsaid batches having an isotope concentration substantially equal to thatof the said recombined products, such introduction being efiected whilethe hatch into which the introduction is eflected is.

undergoing decomposition. I

2. A process of concentrating isotopes by electrolysis comprising thesteps of simultaneously decomposing a plurality of batches of materialoi difiering isotope concentration, recombining one of the decompositionproducts of one of said batches to form material substantially equalv inisotope concentration to another of said batches,

vand introducing said recombined material into the latter batch while itis undergoing decompoof diflering isotope concentration, recombining

